Calathium is an extinct genus of organism found in marine beds of Ordovician age.

Its classification is enigmatic: It has long been placed among the receptaculites,[1][2] but it has also been described as a quasi-sponge,[3] possibly akin to the archeocyathids[4] or other hypercalcified sponge.

[5] The chief difference from archaeocyathids is that their walls were connected by rods rather than septae.

[4] The organisms were important reef-forming organisms during the Ordovician, forming communities with lithistid sponges that gradually displaced the earlier microbial mounds.

This poriferan- (or sponge-) related article is a stub.